1. Field of the Invention
The present invention relates to an arrangement in a digital communications network for transmitting synchronously transmittable first information items, for example speech, video and/or data, and asynchronously transmitted second information items, for example data.
2. Description of the Related Art
The invention can be used, inter alia, in connection with the so-called "hardware platform" corresponding to the longitudinal layer of the OSI model, in which, especially, the MAC (Medium Access Control) part is of interest. The invention also takes into account the existing physical layers and utilization of current software which is included. The invention can be used, inter alia, with a DTM (Dynamic Synchronous Transfer Mode) protocol such as that used in the MultiG project and; to the proposed IEEE 802.9.
Circuit switching has traditionally been considered to be good for transmitting speech. In digital communication, synchronous and asynchronous multiplexing are known. In synchronous multiplexing, the information is transmitted in channels which consist of time slots which are permanently allocated in advance. The communication is connection-oriented, that is to say must be preceded by a call set-up. A time interval can contain time slots for different channels. One cycle of recurrent time slots is called a frame. This synchronous multiplexing is one way of implementing circuit switching.
In connection with the integration of speech, video and data in local area networks, (LAN) it is quite generally known to produce dynamic bandwidth allocation. Bandwidths which are not used for speech and image communication can be used for asynchronous data communication. It is known per se to utilize a system of the token-ring type and message coding with differential Manchester coding. In connection with this, it has been proposed to utilize the token-ring system in a time-division multiplexed (TDM) connection.
Reference is made to, among others, American patent specification U.S. Pat. No. 4,866,704 which describes an asynchronous fiber optical local area network. The network supports data packet traffic together with synchronous voice traffic over a common token-ring channel.
From American patent specification U.S. Pat. No. 4,843,606 it has previously been known to utilize a communications system in local connection with the token-ring principle. Synchronous bandwidth management is utilized for giving prioritized functions for quasisynchronous frames with regular intervals. The "rings" are mutually connected through a time-division multiplex unit via its synchronous bandwidth manager. Buffers are arranged in each synchronous bandwidth manager for synchronous information blocks transmitted from and to a respective ring. A TDM control unit utilized can independently reach the said buffers for TDM rerouting via individual bytes in the information block which can consist of voice information. The buffer arrangement can include FIFO (First-in-First-out) buffers. The rings are also mutually connected within a "backbone" bus or ring for transmitting asynchronous data between the rings. The number of rings can be one or more.
Reference is also made to American patent specification U.S. Pat. No. 4,785,448, which relates to a local area telephone system for simultaneous transmission of digital data and analog voice signals on the same transmission medium. The station units are physically connected in a star configuration. Token-ring transmission is also utilized. Moreover, Manchester coding is utilized.
Reference is also made to American patent U.S. Pat. No. 4,553,234 which relates to a broadband local area network with token-ring transmission and time-division multiplex in both circuit-switched and packet-switched traffic. In connection with the known arrangement, data, image and speech traffic is transmitted, among others.
In American patent specification U.S. Pat. No. 4,459,558, a token-ring protocol for a local area network is utilized. A ring binds together a plurality of stations for forming a local area network. Each respective station is allocated one or more of three priority levels corresponding to service types. The highest level guarantees a bandwidth, for example for digital voice data. The second level relates to interactive data communication without absolute bandwidths guarantee. Level 3 relates to low-priority transmission.